Hand tools such as chisels are conventionally given a good cutting edge by hand-honing on an oil-stone. However from time to time more radical grinding is necessary e.g. to remove nicks in the sharpened edge and to re-establish the conventional 25.degree. angle between the two faces at the edge. This can be done by using a surface grinder with a cup-shaped grinding wheel and a carriage for holding the hand tool which is reciprocated to and fro across the axial annular working face of the grinding wheel by a chain e.g. for a distance of two feet. Such surface grinders are expensive and non-portable (they have cast iron beds) and they can take a long time to remove perhaps 1-2 thou (25 to 50 micrometers) from the sharpened edge due to the length of travel. Alternatively a portable grinder with a standard disc grinding wheel can also be used but such a grinder is not provided with a carriage and the operator has to be skilled if he is to provide a straight sharpened edge in the minimum time without overheating the steel so that its temper is lost.
There is therefore a need for a relatively inexpensive adapter apparatus for a bench grinder which will permit safe, rapid, accurate, automatic grinding of a tool or blade near where it is being used rather than in an engineering workshop.
A tool holding jig is known from GB 378162 for use when grinding chisels and plane blades. This jig comprises a pair of linkages arranged in a parallelogram configuration by each having one end universally pivotally mounted on a base of a grinding machine and by each having near its other end a uniaxially pivotal connection to a tool holding member onto which a tool can be clamped. In use, the tool is rocked to and fro across the grindstone manually using handles at said other ends of the linkages. A problem with this known jig is that the angle to which the tool is ground is dependent on the position in which the tool is clamped to the holding member, and thus care needs to be taken when mounting the tool in the holding member to ensure the correct angle will be achieved. A further problem with this known jig is that the type of pivotal connections which are shown and described will enable the parallelogram configuration to skew so that the difficulty will be caused when attempting to grind a straight square edge.